1. Field of the Invention
The present invention generally relates to the generation and phase control of a pixel clock for use in laser printers, digital copiers, and other image forming apparatuses in general, and particularly relates to a pixel-clock generating circuit and an image forming apparatus having such a circuit wherein the pixel-clock generation circuit achieves highly precise phase control of a pixel clock.
2. Description of the Related Art
FIG. 25 is an illustrative drawing showing a general construction of an image forming apparatus such as a laser printer or a digital copier. In FIG. 25, a laser beam emitted by a semiconductor laser unit 1001 is scanned by a rotating polygon mirror 1002, and forms a light spot through a scan lens 1003 on a photoconductor 1004 serving as a medium to be scanned, thereby forming a latent image through the exposure of the photoconductor 1004 to light. For this purpose, a clock generating circuit 1008 generates a clock signal. Based on this clock signal, a phase synchronizing circuit 1009 generates an image clock signal (pixel clock) having synchronized phase with respect to each line in response to an output signal of a photo-detector 1005. The image clock signal is supplied to an image-processing unit 1006 and a laser drive circuit 1007. With this provision, the laser drive circuit 1007 controls the timing of laser emission according to image data generated by the image-processing unit 1006 and the image clock signal having its phase controlled by the phase synchronizing circuit 1009 for each line, thereby controlling the latent image on the photoconductor 1004.
In this optical scan system, the deflection surfaces of a deflector such as a polygon scanner may have varying distances from the center of rotation. Such variation causes fluctuation of scanning speed as the light spot (scan beam) scans over a target surface. The fluctuation of scanning speed results in fluctuation of images, causing degradation of image quality. When high image quality is required, thus, the fluctuation of scanning needs to be corrected. In the case of a multi-beam optical system, a difference in the wavelengths of light sources results in the error of exposed positions where the optical system is not corrected for chromatic aberration of the scan lens. The width of scanning when the light spots of respective light sources scan over the target medium differs for each light source, causing degradation of image quality. This gives rise to a need for the correction of scan widths.
As a conventional technology for correcting the fluctuation of scanning or the like, the position of a light spot is controlled along a scan line by changing the frequency of a pixel clock as disclosed in Patent Document 1 and Patent Document 2.    [Patent Document 1] Japanese Patent Application Publication No. 11-167081    [Patent Document 2] Japanese Patent Application Publication No. 2001-228415
However, the conventional scheme that changes the frequency of a pixel clock has a drawback in that the construction of a pixel-clock control unit becomes excessively complex, and that such complexity increases as the step of frequency modulation becomes finer. This prohibits diligent frequency control.
Accordingly, there is a need for a pixel-clock generating circuit and an image forming apparatus having such a circuit wherein pixel-clock generating circuit has a simple construction for achieving the phase control of a pixel clock.